The use of porcelain in the manufacture of dental prosthesis such as caps, bridge work, etc. is well known in the art. Dental grade porcelain products provide strong durable dental restorations which can closely match natural teeth in appearance. In the creation of dental prosthesis, it is well known to create a selectively configured mold dividing it into upper and lower flask portions. Dental prosthesis mold are generally made from investment material such as V.H.T. Industrial Investment available from the Whip Mix Corp., Louisville, Ky. If the porcelain can be pressed into the mold without problems, the fabrication of porcelain prosthetic appliances can be made easily and efficiently.
In conventional porcelain press techniques, it is common to use porcelain powder.
First, the investment is degassed. This process entails drying the mold to release ammonia gas. If the mold is not degassed, ammonia or other gas may be released during the pressing process which can cause bubbles to form within the molded prosthesis. After degassing, the lower flask is soaked in water to permit the investment to absorb sufficient water. Next, the water/porcelain powder mixture is poured into the mold and then condensed by vibrating the flask. Excess water coming up to the surface by vibration may be removed by blotting with tissue paper. The process is repeated until excess water does not emerge. After water is completely removed from the porcelain and investment; the flasks are placed in the muffle of a furnace; the upper and lower flasks being assembled in preparation of the pressing operation. Porcelain powder is fired according to the porcelain manufacturer's recommended guidelines. The temperature is then increased (about 100.degree. C. higher than firing temperature) so as to soften the porcelain under air pressure. The flask halves are then closed pressing the porcelain into the mold cavity.
Users regarded this method as unsatisfactory for porcelain press technique, because of the following:
porcelain powder in the mold may not be condensed properly, because water from the mixture is likely to be absorbed into the investment during the procedure of condensation;
porcelain powder may be dispersed by steam or gases grom the mold while it is being dried;
removal of water from porcelain powder and investment is very time consuming;
white color may appear in the porcelain around the porcelain excess area after pressing;
discoloration and bubbles may occur in the porcelain, brought about by steam and gases during the process; and
uneven baking of the porcelain may occur within the mold during the firing.
The significant difference in the degree of softness in the porcelain at the surface of the mold and within the interior of the mold, while the firing is being performed, is a contributing factor towards unsatisfactory pressing of porcelain into the required detail.
From the above, it is seen that porcelain powder used in porcelain press techniques creates unnecessary problems leading to inefficiency in time and labor. Numerous attempts have been made in the past to find a satisfactory solution to the problems, but without success.